1. Field of the Invention
This invention relates to a control system for an internal combustion engine and, more particularly, to an engine control system for controlling an air-to-fuel ratio leaner than an ideal combustible air-to-fuel ratio.
2. Description of Related Art
In order for automobile engines to improve fuel economy, an air-to-fuel ratio is controlled to be leaner than an ideally combustible air-to-fuel ratio while the engine has been warmed up and operates with light engine load. Such an air-to-fuel control, known as a lean control, is known from, for instance, Japanese Unexamined Patent Publication No.59-208141.
In the case of conducting a lean control while the engine has been warmed up and operates with a light engine load, the air-fuel mixture setting is suddenly varied to be lean after having been maintained at an ideally combustible air-fuel mixture setting for a while. During the transition of air-fuel mixture setting to a leaner air-to-fuel ratio, a reduction in the amount of fuel delivered by an injector occurs. Such a fuel reduction leads to a temporary engine torque drop and, hence, adversely affects the driving performance of the automotive vehicle. In this instance, it has been proposed to supplementally cancel the torque drop caused due to a fuel reduction through what is referred to as an "enrich control" in which intake air is introduced by the aid of an idle speed control valve (ISC valve), for allowing and regulating intake air to bypass a throttle valve, so as to enrich a fuel mixture when the lean control is conducted. However, only conducting the lean control accompanied by the enrich control during the transition of air-fuel mixture setting to a leaner air-to-fuel ratio causes a temporary drop in output torque of the engine due to the difference between responsiveness of fuel flow and air flow. For example, when the lean control commences during acceleration, fuel flow increases its flow rate with a relatively high responsiveness and air flow is sluggish in responsiveness. On the other hand, when the lean control commences during deceleration, since air is introduced into the engine due to an inertial flow of intake air and/or a Stagnation downstream from the throttle valve at the beginning of deceleration in addition to an increase in amount of air provided by the enrich control, a feeling of deceleration becomes poor. The lean control is ordinarily conducted not during a cold period in which the stability of fuel combustibility is possibly lowered but after warming-up. However, if an abrupt increase in the amount of intake air is caused by the enrich control, at the commencement of the lean control when the lean control is conducted, as a result of a transition to a warm-up condition from a cold condition while the engine operates in a range of lower engine loads, the engine output torque will suddenly jump up, so that a so called "torque shock" is experienced. This is because the air charge rate increases before the air-to-fuel ratio of a fuel mixture becomes lean.